1. Field of the Invention
The present invention relates to a quartz glass vessel adapted for use in a process for the growth of single crystal material such as a single crystal silicon material.
2. Description of the Prior Art
Conventionally, in producing a single crystal such as a single crystal silicon semiconductor material, the so-called "Czochralski" method has been widely adopted. According to the method, polycrystalline silicon is molten in a vessel and a seed crystal is dipped at its tip end portion into the melt. Then, the seed is pulled while it is being rotated so that a single crystal is grown on the seed with the same crystallographic orientation.
As the vessel adapted for the pulling of a single crystal, use is made of a quartz crucible. The quartz crucible is classified in accordance with the external appearance entailed by the difference in the production methods into a transparent crucible having a relatively low bubble content and an opaque crucible having a high content of minute bubbles to provide an opaque appearance. The latter crucible will hereinafter be referred to as the "opaque crucible".
The opaque crucible is produced by a process in which powders of quartz are introduced into a mould to be accumulated to form a layer along the inner surface of the mould. The layer of the quartz powder is then heated at the inner surface thereof while the mould is being rotated to produce a crucible of a glass having a relatively high bubble content. The opaque quartz crucible is advantageous in that it has a higher strength as compared with a transparent crucible and it is relatively easy to form a crucible of a large size. Further, minute bubbles contained in the opaque crucible contribute to uniformize the thermal distribution. From the reasons stated above, the opaque crucible has widely been used in practice.
It should however be noted that with the conventional crucible of opaque type it has been experienced in the process of producing a single crystal that crystallization becomes unstable with the result that the yield is decreased. There may be several reasons for the instability of crystallization. Among the reasons for the instability, one which is caused by the crucible is that the inner surface of the crucible is eroded due to a reaction under an elevated temperature between the silicon melt and the quartz glass so that irregularities are produced in the inner surface of the crucible. As the erosion further progresses, the bubbles which have been contained in the crucible are exposed to the silicon melt. When the irregularities are thus produced in the location where the crucible is kept in contact with the silicon melt, the surface of the melt cannot be lowered smoothly as the quantity of the melt decreases causing fluctuations of the melt surface. It should further be pointed out that minute projections produced in the roughened surface of the crucible can become nucleus of crystallization of quartz glass forming spotty crystobalites on the quartz crucible. The crystals thus formed may be separated from the crucible as the pulling process progresses and fallen into the silicon melt. The crystals thus fallen into the silicon melt will have an adverse effect on the growth of the crystal which is being pulled.
A further problem is that metallic impurities may be concentrated in the vicinity of the inner surface of the quartz crucible, for example, at the surfaces of the minute bubbles and the concentrated metallic impurities are heated in the process of pulling the crystal to thereby enhance the crystallization of the crystobalites producing spotty crystobalites. From similar reasons, it is not preferable that the crucible has minute projections and scratches at the time when the crucible is formed.
In order to solve the aforementioned problems experienced in the conventional quartz crucible, the Japanese laid-open patent application 59-213697 teaches to form an opaque quartz crucible produced as described above with a layer of transparent quartz glass of at least 1 mm thick on the inner surface of the crucible at least at a portion where the crucible is brought into contact with the melt. Such a layer of transparent quartz glass is formed by heating the inner surface of an opaque quartz glass crucible for a long time period. It should however be noted that the transparent quartz glass layer formed in accordance with the teaching of the laid-open patent application is not satisfactory because the bubble content is not sufficiently low but still contains a certain amount of minute bubbles. According to recent technologies for pulling a single crystal, the process is carried out under an atmosphere of a reduced pressure and the quartz crucible formed in accordance with the teaching of the Japanese laid-open patent application has a tendency that the bubbles in the crucible are expanded under a reduced pressure which the crucible is subjected to during the single crystal pulling process. Thus, problems are encountered even in this type of crucible as experienced in the conventional crucible.
The U.S. Pat. No. 4,528,163 teaches to form a crucible substrate with an outer layer made of powders of natural quartz and an inner layer made of powders of synthetic quartz. The crucible substrate is then heated at the inner surface to form a thin smooth layer of an amorphous nature. However, as in the case of the transparent glass layer of the Japanese laid-open patent application, the amorphous layer formed in accordance with the teaching of the U.S. patent contains noticeable quantity of bubbles or voids.
The U.S. Pat. Nos. 4,416,680 and 4,632,686 teach to decrease the bubble content by applying a suction pressure to the external surface of the quartz crucible while the crucible is being heated. It should however be noted that the process as taught by the U.S. patents are not effective to decrease the bubble content because the bubbles are subjected to a substantial resistance in passing through the layer of quartz glass.
It should further be noted that in the process of pulling a single crystal it is required in addition to the stabilization of the pulling operation to maintain an accurate control of oxygen which is transferred from the crucible to the single crystal which is being pulled. However, in the quartz crucible made in accordance with the conventional process, the inner surface is apt to be roughened in the pulling operation due to the fact that the inner layer of the crucible is not satisfactorily uniform and that the crucible contains substantial quantity of bubbles. Therefore, the quantity of the quartz which is molten from the crucible into the melt becomes instable and it becomes difficult to accomplish the requirements of accurate control of the oxygen transfer.
A further problem encountered in the conventional quartz crucible is that the crucible is apt to be locally deformed during the pulling operation when the crucible is subjected to a high temperature for a long period of time. The technologies as taught by the Japanese laid-open patent application and the aforementioned U.S. patents are not effective to solve the problem.